Method for manufacturing multiple channel membranes, multiple channel membranes and the use thereof in separation methods

ABSTRACT

A method for manufacturing multiple channel membranes includes extruding a polymer solution through an extrusion nozzle wherein several needles are arranged through which a gas or liquid containing a coagulating agent is injected during extrusion, wherein the outer side of the extruded material is first brought into contact with a mild coagulation agent and subsequently with a strong coagulation agent. By using the method a membrane is obtained having parallel channels extending in extrusion direction, in which an active layer is situated in the channels, whereas the outer surface with respect to the active layer has no or hardly any resistance to flows of liquid. By using the method it is possible to make shapes, such as recessed portions, in the membrane circumference. Also disclosed is the use of such membranes in filtration and separation techniques.

The present invention relates to a method for manufacturing multiplechannel membranes by extrusion of a solution of a polymer which can forma semi-permeable membrane after coagulation. The invention furtherrelates to membranes that can be obtained using this method and to theuse of said membranes in separation, filtration and purificationtechniques.

Membranes of semi-permeable materials that are provided with severalcontinuous channels are known.

FR 2,616,812 A relates to a method for manufacturing a porous organicmaterial, particularly an organic semi-permeable membrane, by extrudinga solution of a polymer and coagulating it, to an extrusion nozzle forcarrying out said method, to the membranes obtained and to filtrationmodules containing such membranes. According to FR 2,616,812 A a polymersolution is extruded through an extrusion nozzle which is provided withseveral separated pipes through which a liquid is injected so that anextrudate is formed having several longitudinal channels, andsubsequently coagulation is carried out in order to form the porousorganic material. By using a non-solvent for the polymer as liquid to beinjected, and guiding the extrudate immediately after leaving theextrusion nozzle into a bath containing a non-solvent, an active layerconsisting of small pores is formed both in the channels and on theouter surface of the membrane. According to FR 2,616,812 A by firstrunning the extrudate through an air gap prior to guiding it into a bathwith a non-solvent, a membrane is obtained having only an active layerin the channels, and by injecting a liquid which does not precipitatethe polymer, and guiding the extrudate immediately after leaving theextrusion nozzle into a bath of non-solvent, a membrane having an activelayer on the outer surface is obtained. The multiple channel membranesof this reference may for instance be flat or cylindrical.

EP 0,375,003 A1 and EP 0,375,004 A1 relate to the manufacturing oforganic semi-permeable membranes provided with several separatedchannels by means of the method of FR-2,616,812 A. Said referencesdescribe in particular the dimensions of the extrusion nozzles, theneedles present in them, the dimensions of the channels and the wallthickness of the extruded membrane, the viscosity and the volume of thepolymer solution to be extruded and of the injected liquid and thelength of the air gap.

The larger mechanical strength, the easy handling and the higherproduction speed as well as the easy use in filtration modules, arementioned in FR 2,616,812 A as advantages of the multiple channelmembranes with respect to the known hollow-fibre membranes.

FR 2,437,857 A relates to cellulose dialysis membranes in the shape ofhollow fibres, in which two or more hollow fibres are connected to eachother parallel to the fibre axes. Said membranes are obtained by usingan extrusion nozzle provided with conduits through which a liquid isinjected which forms the channels.

WO 81/02750 relates to the manufacturing of a membrane unit of asemi-permeable synthetic material provided with a number of paralleltubular channels wherein the synthetic material is extruded through anextrusion nozzle which is provided with a number of thin metal threadsor a number of conduits through which a liquid is injected.

DE 3,022,313 A1 relates to multiple hollow fibres, in which the hollowfibres have several separated cavities which extend in the length of thehollow fibre. The multiple hollow fibres are made either by adhering anumber of hollow fibres having one cavity to each other, or by extrudinga hollow fibre having several cavities, preferably no more than four.The hollow fibres are intended for dialysis.

Above-mentioned dialysis membranes, particularly membranes for kidneydialysis consist of cellulose derivatives. The characterizing feature ofsaid membranes is that the membrane wall is homogeneous and therefore initself responsible for the resistance against liquid permeability.Because of this the wall is made as thin as possible, usually in theorder of 0.15 μm. Because in dialysis no or hardly any pressuredifference is exerted over the membrane said thin wall is no problem. Infor instance ultra-filtration and micro-filtration there is indeed apressure difference and the membrane will have to be able to resist apressure of at least 3 bar. The thin walls of dialysis membranes are notresistant to such a pressure.

The known semi-permeable membranes are guided into a coagulation bathafter extrusion either directly or after running through an air gap. Inthe first case a separating layer is always formed at the outer surfaceof the membrane in addition possibly to a separating layer formed in thechannels. Using an air gap makes it possible that a membrane is formedwhich only has a separating layer at the channel side. The length ofsaid air gap should be such that the structure of the membrane issufficiently fixed by the coagulation liquid which diffuses from thechannels into the extruded membrane material, before the membrane entersa coagulation bath for further removal of the soluble components.Because of the length of the air gap the membrane can sag as a result ofits own weight while it is still in a substantially liquid condition. Asa result it is necessary to use polymer solutions of a high viscosity,such as for instance is described in FR 2,616,812 A, EP 0,375,003 A1 andEP 0,375,001 A1. In order to obtain a polymer solution of a highviscosity, a high concentration of polymer and/or polymer additives areused. As a result the coagulation is slowed down whereas said additivesare hard to rinse out. A high concentration of polymer in the solutionalso gives a membrane having a low flux. Moreover a longer air gap maycause a shape made in the outer surface to disappear due to flow underthe influence of surface tension.

By using the methods described above it is not possible to manufacture amembrane of a complex shape, such as a flat multiple channel membranehaving recessed portions parallel to the channels, in which an activelayer is formed in the channels only.

Methods in which coagulation from one side is effected so that themembrane structure is fixed before the membrane reaches the coagulationbath, suffer from the drawback that no larger wall thicknesses can beproduced so that the diameters of the channels are strongly limited.

An object of the invention is therefore to provide a method for themanufacturing of multiple channel membranes which do not entail theabove-mentioned drawbacks.

Said objective is achieved according to the invention by a method formanufacturing multiple channel membranes, wherein a solution of apolymer which forms a semi-permeable membrane after coagulation, isextruded through an extrusion nozzle wherein several hollow needles arearranged, a gas containing coagulating vapour or a coagulating liquid isinjected through the hollow needles into the extruded material duringextrusion, so that parallel continuous channels extending in extrusiondirection are formed in the extruded material, and the outer surface ofthe membrane is brought into contact with coagulation agents,characterized in that the outer surface of the membrane after it leavesthe extrusion nozzle is first brought into contact with a mildcoagulation agent such that the shape of the membrane is fixed withoutan active layer being formed on the outer surface of the membrane andsubsequently the membrane is brought into contact with a strongcoagulation agent.

By using the method according to the invention it is possible to controlthe pore size on the outer surface of the membrane and those in thechannels independent from each other. As a result a membrane can beobtained having a separating layer in the channels in which the outersurface with respect to the active layer has no or hardly any resistanceagainst liquid flows in for instance micro- or ultra-filtration.

In the method according to the invention coagulation takes place fromtwo sides, which results in the coagulation distances being reduced upto a factor two.

The distance above the coagulation tank where the partly liquid membranehas to hang from itself becomes much smaller because the largest part ofthe coagulation takes place in the coagulation/rinse bath. In thecoagulation bath the difference in specific weight between the membraneand the bath is very small in case of usual polymers and solvents. Thecoagulation path (residence time) in such a bath can be chosen to be aslong as necessary. As a result also thin viscous solutions can be spun.It appeared that by using the methods of the present invention multiplechannel membranes can be formed from the low viscous polymer solutionsaccording to WO 99/02248, which according to said reference are onlysuitable for the manufacturing of flat membranes on carriers and not forthe manufacturing of capillary membranes. In a membrane obtained withsuch a thin solution only low molecular substances are present that caneasily be removed.

With the method of the invention it is possible to make shapes, such asrecessed portions parallel to the channels having a larger cross-sectionin the outer circumference of the membrane.

According to a preferred embodiment of the method according to theinvention a solution, for which water is a non-solvent, is brought intocontact with a vapour having a relatively high water vapour tension asmild coagulation agent, after leaving the extrusion nozzle.

In this vapour path some water diffuses into the outer layer of theextruded material so that at that location superficial separation occursand a coarser pore structure is formed. Then the membrane is submersedin water as a result of which the structure of the membrane is fixed.

According to another embodiment a mild coagulation agent is applied onthe extruded material by means of an additional outlet on thecircumference of the extrusion nozzle.

By bringing the extruded material in contact with a mild coagulationagent both in the channels and on the outer surface, it is possible toobtain a membrane having an active layer consisting of micro-pores bothin the channels and on the outer surface, in which between said activelayers a layer having larger pores is situated.

The invention further provides membranes obtained by using the method ofthe invention.

A preferred embodiment of the membranes according to the inventioncomprises a flat multiple channel membrane having recessed portionswithout channels extending parallel to the channels, in which theseparating layer is arranged in the channels and the outer surface withrespect to the active layer has no or hardly any resistance againstflows of liquid.

Such a membrane is particularly suitable for use in spiral woundelements as described in U.S. Pat. No. 4,756,835. Because of thepresence of the recessed portions without channels the flat membrane ofthe invention is less rigid than the known flat membranes and is lessresistant to rolling up. Flat multiple channel membranes have a certainrigidity and when rolling up the membrane it obtains a curvature radiusbecause of which, as a result of the shape of the membrane, a tensilestress arises on one side and a compression arises on the other side, asa result of which channels can be deformed and the pores can beinfluenced. It appeared that already with a limited number of recessedportions a membrane that can be rolled up well can be obtained. Contraryto the membrane of U.S. Pat. No. 4,756,835 which is built up frommembrane sheets having grooves that are placed against each other, thepresent membrane is extruded in one go. As a result circular channelscan easily be obtained. In order to achieve that in the membranes ofU.S. Pat. No. 4,756,835, the membrane sheets have to be laid on eachother with great precision, which is a problem with larger sheets. Whenmanufacturing a spiral wound element this becomes even worse, becauseway length differences then occur between inner membrane sheet and outermembrane sheet, as a result of which the grooves shift with respect toeach other. The consequence is that the optimal flow pattern isdisrupted and dead cavities are formed. With respect to the membranes ofU.S. Pat. No. 4,756,835 the present membranes have the further advantagethat no delamination of the sheets leading to large leakage flows willoccur.

A spiral wound membrane having the active layer in the channels has theadvantage that it is possible to make an element having a capillarymembrane in the much quicker and more efficient way used for capillarymembranes, whereas the better defined flow of a capillary element ispreserved.

Another preferred embodiment of the membrane according to the inventionis a cylindrical multiple channel membrane in which the active layer isarranged in the channels, in which the surface area of the channels ismore than 1.5 times the outer surface area and the outer surface withrespect to the active layer in the channels has no or hardly anyresistance to flows of liquid. A cylindrical membrane having a largerdiameter and a large number of channels can be mounted in a hollow fibreelement considerably easier and is mechanically more stable than anumber of single hollow fibre membranes having the same channel size. Ina cylindrical membrane having a large number of channels the ratiobetween the total channel surface area and the outer surface area islarge. This is no problem in the membranes according to the inventionbecause the active layer is situated in the channels. In case therewould also be an active layer on the outer surface, the resistanceagainst liquid flows is considerable.

Because a membrane having several channels is extruded in one go, alarger mechanical stability is obtained with respect to single channelshaving a same channel size.

As a result of the larger mechanical stability, the membranes accordingto the invention are particularly suitable for cleansing by backwashing. That means that the filtration direction is periodicallyreversed so that a possible fouling layer formed in the channels islifted and can be removed. Said technique is mainly used inultra-filtration and micro-filtration.

The membrane material is preferably a soluble thermoplastic polymer.Suitable polymers are known to the expert. Examples are polysulfones,poly (ether sulfones), polyvinylidene chloride, polyvinylidene fluoride,polyvinyl chloride, polyacrylonitrile, etc. The polymer is dissolvedprior to extrusion in a usual solvent and additives can be added. Ausual solvent is N-methylpyrrolidone.

Coagulation agents are known to the expert. Many used coagulation agentsare non-solvents for the polymer that are miscible with the solvent. Thechoice for the non-solvent depends on the polymer and the solvent. Asolvent used much is N-methylpyrrolidone. Examples of non-solvents foruse with this solvent are dimethylformamide, dimethyl sulfoxide andwater. The strength of the coagulation agent can be adjusted by thechoice of the combination solvent/non-solvent and the ratiosolvent/non-solvent. The coagulation can also be performed with a liquidthat is not related to the solvent.

It is also possible to form a separating layer by applying a coating inthe channels. Coating materials usual to that end are known to theexpert. A survey of suitable coating materials is given by Robert J.Petersen in Journal of Membrane Science 83 , 81-150 (1993).

The diameter of the channels of the multiple channel membranes of theinvention is between 0.1 and 8 mm and preferably between 0.1 and 6 mm.The thickness of the walls is adjusted to the pressure to be exerted inthe channels depending on the intended use, such as for instancemicro-filtration, ultra-filtration, nano-filtration, gas separation andreverse osmosis. In general the thickness of the walls is between 0.05and 1.5 mm and preferably between 0.1-0.5 mm. The cylindrical membranescontain at least four and preferably 7 to 19 channels. The diameter ofthe cylindrical membrane generally is between 1 to 20 mm and preferablybetween 2 and 10 mm.

The locations of the recessed portions, which according the inventionare provided in the flat membranes so that they can be rolled up better,depend on the wanted curvature radius. Because in a spiral wound elementthe curvature radius near the axis is smaller than further removed fromthe axis, fewer recessed portions may be made in the portion that isfurther removed from the axis than in the portion close to the axis.Preferably recessed portions are made at the edges of the membrane toprevent deformation of the outer channels. Recessed portions arepreferably made opposite each other in the upper and lower surface ofthe membrane. The depth of the recessed portion is generally between 10and 45% of the membrane thickness, for instance between 20 and 40%, andits width is between 0.5 and 6 times and preferably between 1 and 3times the channel diameter.

FIG. 1 shows a schematic view of the cross-section of a flat membranehaving recessed portions according to the invention. In FIG. 1, 1 refersto the membrane, 2 refers to a channel and 3 refers to a recessedportion. FIG. 2 schematically shows a cross-section of the structure ofthe membrane around a channel. In FIG. 2, 1 refers to the membrane, 2refers to a channel, 4 refers to the active layer which is arranged inthe channels and 5 refers to the layer of controlled pore size on theouter surface, which with respect to the active layer in the channels,has no or hardly any resistance to flows of liquid.

EXAMPLE 1-FLAT MEMBRANE

A polymer solution of 20% poly (ether sulfone) (Amoco Radel A100), 9%polyvinylpyrrolidone (PVP) (ISP, K90), 10% glycerin and 61%N-methylpyrrolidone (NMP) was extruded through a rectangular extrusionnozzle having a width of 200 mm and 160 needles of 0.8 mm and at thelocation of the needles having a thickness of 1.2 mm, provided withthree elevated portions having a thickness of 0.4 mm and a length of 2mm on the positions 10, 50 and 100 mm from the edge.

A solution of 40% NMP in 60% water was injected through the needles as aresult of which channels were formed in the polymer solution. Thediameter of the channels was 0.9 mm, the thickness on the portionshaving channels was 1.3 mm and the recessed portions were 0.4 mm thick.

The extrusion speed was 7 m/min, the coagulation bath had a temperatureof 80° C. and the length of the path through vapour was 20 cm. (watervapour having a relative humidity of 80 to 100% at 60° C.)

After rinsing and removal of the superfluous PVP a membrane was obtainedhaving a flux of 1350 l/m²/h/bar (in relation to the channels). Thecut-off value was 120,000 D. The pores in the outer surface were 2micron.

The membrane sheet was very well flexible on the notches and suitablefor spiral wound manufacturing.

EXAMPLE 2-Flat membrane

In the same way as in example 1 a membrane was extruded, but now with52% NMP in 48% water as the injection liquid. After treatment a membranehaving a flux of 2500 l/m²/h/bar and a pore size of 0.1 micron wasobtained. The pores in the outer surface were 2 micron. This membranesheet as well was suitable for spiral wound manufacturing.

EXAMPLE 3-Cylindrical membrane

A polymer solution of 20% poly (ether sulfone) (Amoco Radel A100), 9%polyvinylpyrrolidone (ISP, K90), 10% glycerin and 61%N-methylpyrrolidone (NMP) was extruded through an extrusion nozzlehaving a diameter of 3.4 mm and 7 needles of 0.8 mm.

A solution of 40% NMP in 60% water was injected through the needles as aresult of which channels were formed in the polymer solution. Thediameter of the channels was 0.9 mm, the total diameter was 3.4 mm.

The extrusion speed was 7 m min, the coagulation bath had a temperatureof 80° C. and the length of the path through water vapour was 20 cm.

After rinsing and removal of the superfluous PVP a membrane was obtainedhaving a flux of 1400 l/m²/h/bar (in relation to the channels). Thecut-off value was 125,000 D. The pores in the outer surface were 2micron.

EXAMPLE 4-Cylindrical membrane

In the same way as in example 3 a membrane was extruded, but now with52% NMP in 48% water as the injection liquid. After treatment a membranehaving a flux of 3000 l/m²/h/bar and a pore size of 0.1 micron wasobtained. The pores in the outer surface were 2 micron.

EXAMPLE 5-Cylindrical membrane

A polymer solution of 15% poly (ether sulfone) (Amoco Radel A100), 38%propionic acid and 47% N-methylpyrrolidone was extruded through theextrusion nozzle as used in example 3. The solution had a viscosity ofapproximately 100 cP. A solution of 10% NMP in 90% water was injectedthrough the needles, as a result of which channels were formed in theextruded polymer solution. The diameter of the channels was 1 mm and thetotal diameter was 4.1 mm. The extrusion speed was 7 m/min, thecoagulation bath had a temperature of 70° C. and the length of the paththrough water vapour was 10 cm. After rinsing a membrane was obtainedhaving a flux of 800 l/m²/h/bar. The cut-off value was 30,000 Dalton.The pores in the outer surface were 0.5 micron.

What is claimed is:
 1. Method for manufacturing multiple channelmembranes, wherein a solution of a polymer which forms a semi-permeablemembrane after coagulation, is extruded through an extrusion nozzlewherein several hollow needles are arranged, a gas containingcoagulating vapour or a coagulating liquid is injected through thehollow needles into the extruded material during extrusion, so thatparallel continuous channels extending in extrusion direction are formedin the extruded material, and the outer surface of the membrane isbrought into contact with coagulation agents characterized in that theouter surface of the membrane after it leaves the extrusion nozzle isfirst brought into contact with a mild coagulation agent such that theshape of the membrane is fixed without an active layer being formed onthe outer surface of the membrane and subsequently the membrane isbrought into contact with a strong coagulation agent.
 2. Methodaccording to claim 1, wherein the mild coagulation agent is watervapour.
 3. Method according to claim 1, wherein the mild coagulationagent is a liquid which is applied on the extruded material by means ofan additional outlet on the circumference of the extrusion nozzle. 4.Method according to claim 1, further comprising providing a separatinglayer by coating on the surface of the membrane in the channels. 5.Method according to claim 1, wherein the extrusion nozzle at thecircumference is provided with elevated portions, so that a membranehaving recessed portions in the outer circumference extending in theextrusion direction, is obtained.
 6. Spiral-wound filtration element,comprising: one or more multiple channel membranes produced by themethod according to claim 1, each membrane in the form of a surface withchannel-free recessed portions extending parallel to the channels, themembranes wound around a central axis and having the channels running inthe direction of the axis of the winding, wherein, an active layer isarranged in the channels, no active layer is provided on an outersurface, and any resistance against liquid flows is predominantlydetermined by the active layer.
 7. Method according to claim 2, furthercomprising providing a separating layer by coating on the surface of themembrane in the channels.
 8. Method according to claim 3, furthercomprising providing a separating layer by coating on the surface of themembrane in the channels.
 9. Method according to claim 2, wherein theextrusion nozzle at the circumference is provided with elevatedportions, so that a membrane having recessed portions in the outercircumference extending in the extrusion direction, is obtained. 10.Method according to claim 3, wherein the extrusion nozzle at thecircumference is provided with elevated portions, so that a membrane hasrecessed portions in the outer circumference extending in the extrusiondirection, is obtained.
 11. Method according to claim 4, wherein theextrusion nozzle at the circumference is provided with elevatedportions, so that a membrane having recessed portions in the outercircumference extending in the extrusion direction, is obtained. 12.Method according to claim 1, wherein the hollow needles are arrangedwithin a circular extrusion nozzle so that a cylindrical multiplechannel semi-permeable membrane is formed.
 13. Method according to claim12, wherein four or more hollow needles are arranged within the circularextrusion nozzle.
 14. Method according to claim 1, wherein the needlesare arranged in a row within a rectangular nozzle so that a flat sheetmembrane is formed.
 15. Method according to claim 5, wherein the needlesare arranged in a row within a rectangular nozzle so that a flat sheetmembrane having recessed portion without channels extending parallel tothe channels is formed.
 16. Method according to claim 15, furthercomprising the step of winding the flat sheet membrane spirally round acentral axis and placing the wound membrane in a housing, to produce aspiral-wound membrane.